Nitrogen oxides (NOx) emitted from power station boilers, gas turbines, industrial boilers, incinerators, diesel engines, or the like are a major cause of pollution.
NOx is an important air pollutant produced during combustion of fuels. Although the term includes all nitrogen oxides such as N2O, NO, N2O3, NO2, N2O5, NO3, etc., it is used in the present disclosure to refer to NO and NO2, which are the main cause of air pollution.
Methods for inhibiting or reducing NOx production include low excess air firing, combustion zone cooling, combustion air preheating control, combustion device changing, staged combustion, water vapor spray combustion (emulsion combustion), exhaust gas recirculation, fuel conversion and fluidized bed combustion.
Recently, selective non-catalytic reduction or selective catalytic reduction (SCR) is frequently employed for post-treatment of NOx. In selective catalytic reduction, ammonia or urea is sprayed in front of a de-NOx catalyst, such that nitrogen oxide included in the exhaust gas is converted to unharmful water and nitrogen as it passes through the ammonia and the catalyst as follows.4NO+4NH3+O2→4N2+6H2O
This reaction is called standard SCR and is known to exhibit the highest reaction efficiency when the reaction temperature is approximately 300-400° C.
However, as recovery of energy from the exhaust gas from boilers, incinerators or diesel engines is maximized recently, the temperature of the exhaust gas is often below 300° C. Also, incinerators are often operated at 200° C. or below to reduce fuel cost.
As for biomass power plants or glass melting furnaces, a catalyst is disposed in the rear of an electric dust collector or a bag filter to prevent poisoning of the catalyst by Na ions included in the exhaust gas and the catalyst operation temperature is reduced to 200° C. or below.
If the temperature of the exhaust gas is low, the amount of the de-NOx catalyst has to be increased or the efficiency of the de-NOx catalyst has to be improved. However, to increase the amount of the de-NOx catalyst requires additional cost since operation of a blower is necessary because of poor combustion status owing to increased reactor volume and pressure drop. And, to improve the efficiency of the de-NOx catalyst at low temperature is technically very difficult and requires a lot of cost.
Meanwhile, combined cycle power plants produce yellow plume during startup of gas turbines because a large quantity of NO2 is emitted. However, removal using a catalyst is not easy because of low temperature of the exhaust gas and slow rate of the following reaction.    2NO2+4NH3+O2→3N2+6H2O
For this reason, ethanol or other substance is sprayed at the outlet of the gas turbine to remove the yellow plume by removing NO2. However, this method is problematic in that carcinogenic formaldehyde is produced, operation cost increases due to the expensive ethanol and a selective catalytic reduction (SCR) process has to be added since NO, which is produced during normal operation of the gas turbine, is not removed
According to recent studies, it is reported that fast SCR, in which the de-NOx efficiency is the highest at 300° C. or below when the composition of the exhaust gas is NO/NO2=1, is suitable to increase the de-NOx efficiency. The associated reaction is as follows.2NO+2NO2+4NH3→4N2+6H2O
In the fast SCR, it is known that the de-NOx efficiency is the highest at 300° C. or below when the composition of the exhaust gas is NO:NO2=1:1 and the efficiency is up to 10 times that of standard SCR at lower temperatures.
To apply the fast SCR to the actual facilities, the composition of the exhaust gas has to be adjusted to NO/NO2=1 before passing through the de-NOx catalyst.
Since the exhaust gas contains 90% or more NO during normal operation of the boiler, in general, a non-thermal plasma or ozone generator may be used to oxidize NO included in the exhaust gas at 150-200° C., as shown in FIG. 1, so as to convert the NO in the exhaust gas to NO2 and induce the fast SCR reaction.
Although the non-thermal plasma or ozone generator is very useful in that it can be used at low temperature and the degree of NO oxidation can be easily controlled electrically, it is less economical as compared to heating of the exhaust gas to the optimum temperature necessary to achieve the desired reaction efficiency using, for example, a duct burner, at present.
Although cost increase can be avoided if an oxidation catalyst is used to oxidize NO instead of the non-thermal plasma or ozone generator, the performance of the oxidation catalyst is very low at 150-200° C. where the fast SCR reaction occurs effectively. Accordingly, the oxidation catalyst cannot be disposed at the location shown in FIG. 1. In addition, since the oxidation catalyst has to be installed in a fixed manner, the degree of NO oxidation cannot be controlled as desired unlike the non-thermal plasma or ozone generator.